Twin screw extruders in which a mixture of plastic materials is worked at elevated temperature and pressure are well known. One example is disclosed in the patent to Skidmore U.S. Pat. No. 3,082,816, granted Mar. 26, 1963. The system of the Skidmore patent separates solvent by vaporization. A high solvent temperature results from internally generated heat and from external heat supplied from a heating jacket surrounding the housing of the screw conveyor. Such apparatus has proven to be highly successful particularly in systems using solvents that are present in percentages of 60% or less and that have moderate heats of vaporization. However, in handling mixtures containing substantial quantities of water, which has a high latent heat of vaporization, it has been found that very large amounts of heat are necessary, resulting in the use of excessive temperatures which damage some polymers and which also produce high vapor velocities or require an excessive number of evaporative stages. Also, in view of limitations as to heat transfer rate, the material throughput rate must be reduced in order to allow sufficient time to heat the mixture, and this seriously curtails the productivity of the process.
The present invention is addressed to the problem of improving the productivity and the economy of such apparatus and does so by providing for the removal of non-solvent liquid which would tend to volatilize under the conditions of temperature and pressure used in the process, without transferring the latent heat of vaporization.
Briefly, the present invention is directed to an apparatus in which polymeric materials are mixed, worked, and conveyed by a screw conveyor through an elongated housing wherein a high pressure region is produced in the mixture at a point during its travel through the conveyor and a liquid is removed from the mixture upstream of this high pressure region, and wherein another high pressure region is provided upstream of the liquid removal point, which liquid has a relatively high latent heat of vaporization and would tend to volatilize under the conditions of temperatue and pressure present in the apparatus. In accordance with this invention, the outlet means for removing this liquid from the apparatus includes a means for maintaining a high pressure on the liquid so as to maintain it in liquid form as it is withdrawn from the apparatus. A vent with a pressure control valve or the like may be used for this purpose. Thus, it is not necessary to furnish or to transfer heat to satisfy the latent heat of vaporization of the liquid.
In accordance with a preferred form of the invention, the liquid is deliberately introduced into the extruder at a point which is downstream of one high pressure area but upstream of the other high pressure area.
Throughout this specification and in the claims, where reference is made to "upstream" and "downstream", these terms are applied in relation to the direction of flow of the polymeric material in the extruder.
It has been found that the points of introduction and of removal of liquid may be so arranged that the liquid flow is either cocurrent or countercurrent with the flow of the polymeric material.
In any event, it is considered necessary to provide an upstream seal to confine the liquid against upstream escape from the extruder, and a downstream seal to confine the liquid in a manner to prevent its vaporization.
In one preferred form of the present invention the mixture of materials treated comprises an aqueous rubber emulsion and a polymer. Internally generated heat which results from mechanical working of the mixture, together with heat transferred from a steam or hot oil jacket, produce an elevated temperature in the material in the worm flights. Ordinarily, water separated from the mixture would vaporize at this temperature under atmospheric pressure. However, according to this invention, the liquid pressure maintaining means prevents the volatilization of the water which is removed from the mixture and that most or all of its remains liquid.